The present invention relates generally to power converters such as DC-DC power converters, and more specifically to DC-DC power converters configured to provide reduced deadtime.
DC-DC power converters are known for converting an input DC voltage into an output DC voltage having a value smaller or larger than the input voltage value, possibly with opposite polarity and/or with isolation of the input and output ground references. DC-DC converters normally accept input energy from a voltage source at a voltage input, and provide converted output energy at a voltage (and current) output, which is usually a filtered output that operates as a voltage sink.
When isolation is employed in a DC-DC converter, the input voltage is typically switched on and off at a high frequency, and provided to a transformer, which provides the input/output isolation and the suitable voltage conversion. However, because the input voltage is switched at the high frequency, the output voltage and current typically cannot be directly provided to a load in a regulated manner. An inductor is generally required in the energy conversion to act as a current filter. The size and value of the inductor are often critical to meeting the performance specifications. A large inductance volume normally reduces the power density of the converter. Further, because inductors with large inductance values have low slew rates, the response time of the converter to load current disturbances is slowed down. Accordingly, smaller inductance volumes and values are desirable.
Isolated DC-DC converters typically operate with at least some amount of deadtime. For example, a conventional full-bridge converter has deadtime during its operation. Besides preventing switches in the same leg of the converter from conducting simultaneously, this deadtime allows conventional dual-end (e.g., half-bridge, full-bridge, push-pull, etc.) converters to have a regulated output voltage when the input voltage changes.
During the deadtime, the energy into the input is discontinuous, causing a large input current ripple. Large input filters are therefore employed to satisfy conducted Electromagnetic Compatibility (EMC) requirements. This deadtime also necessitates a large output inductor to smooth the output voltage, and to limit the current ripple through it. However, the large output inductor slows the output response time. The volume of the output inductor also takes up valuable board space. Further, as the length of the deadtime increases, the size of the output inductor often increases. Because of this deadtime, simple self-driven synchronous rectification schemes typically cannot be used to achieve high efficiency of power conversion in low voltage, high current output DC-DC converter applications.
Certain topologies produce little or no deadtime, which means that energy is continuously transmitted from the input DC source to the output load during the entire switching period. Other topologies may provide a reduced deadtime. Because the input and output current ripples are generally lower in DC-DC converters having reduced or no deadtime, the input filter is generally smaller. Further, the lower output inductance value improves the output transient speed and reduces the output filter size, thereby improving the power density and output transient response of DC-DC converter. Moreover, the peak to peak voltage ripple across the inductor generally decreases, which allows a reduced inductor volume. Conventional techniques for reducing deadtime include magnetic transformer tapping, and two transformer implementations. However, magnetic transformer tapping typically has manufacturability problems, which can lead to difficulties in transformer operation such as high leakage inductance or magnetic flux imbalance. In addition, the extra switches employed in magnetic transformer tapping can increase losses. Further, the two transformer implementation typically requires an additional magnetic core, which takes up valuable board space. The power density of such conventional DC-DC converter implementations may also be reduced.
Accordingly, there is a continuing need to develop and improve DC-DC converters that operate with reduced or no deadtime.